1. Field of the Invention
The present invention relates to an amplifier circuit for driving a load and a controlling method thereof, a driving circuit of a display apparatus and a controlling method thereof, a portable telephone and a portable electronic apparatus, and in particular, to the amplifier circuit for correcting an offset voltage of an operational amplifier and a driving circuit of the display apparatus for performing multiple gray scale level display.
2. Description of the Prior Art
In the past, an amplifier circuit for driving a load has a problem that an offset voltage arises due to variations in characteristics of active elements constituting the amplifier circuit. To solve this problem, various methods of correcting the offset voltage have been used so far. Of these methods, the amplifier circuits described in Japanese Patent Laid-Open No. 62-261205 and Japanese Patent Laid-Open No. 9-244590 can be named as representative examples of the amplifier circuits having offset voltage correction means using a capacitor.
FIG. 46 is a diagram showing a configuration of the amplifier circuit in the past described in Japanese Patent Laid-Open No. 62-261205. The amplifier circuit in the past shown in FIG. 46 has operational amplifiers 641 and 642 having differential inputs +IN and −IN applied to a non-inverting input terminal and an inverting input terminal from circuit input terminals 621 and 622 respectively, capacitors 631 and 632 and transistor switches 601 to 612. The switches 601, 602, 608, 609, 610 and 611 form a first switch group, and the switches 603, 604, 605, 606, 607 and 612 form a second switch group. The first and second switch groups are controlled to be alternately on.
Operation of the amplifier circuit shown in FIG. 46 will be described. In FIG. 46, first, control is exerted so that the first switch group is in an on state and the second switch group is in an off state. As the switches 601, 602 and 611 are closed in these states, the operational amplifier 641 outputs a differential signal supplied to the input terminal to an output terminal. On the other hand, the non-inverting input terminal of the operational amplifier 642 is grounded, and an offset voltage portion is outputted to the output terminal. The capacitor 632 is charged by this offset voltage so as to hold the offset voltage.
Next, the control is exerted so that the first switch group is in the off state and the second switch group is in the on state. As the switches 606, 607 and 612 are closed and the capacitor 632 is connected in series between the input terminal 622 and the inverting input terminal of the operational amplifier 642 in these states, the differential signal −IN has the offset voltage of a reverse polarity superimposed thereon and is applied to the inverting input terminal of the operational amplifier 642. As a result of this, the output of the operational amplifier 642 has the offset voltage set off therefrom and is corrected.
As the alternate operations of the above switch groups are repeated, the same operation as the operational amplifier 642 is also performed as to the operational amplifier 641, so that the offset voltage of the operational amplifier 641 is also corrected. The corrected output voltages of the operational amplifiers 641 and 642 are alternately outputted to an output terminal 623 so as to allow high-precision output in the amplifier circuit in FIG. 46.
FIG. 47 is a diagram showing the configuration of the amplifier circuit in the past described in Japanese Patent Laid-Open No. 9-244590. The amplifier circuit in the past shown in FIG. 47 has an operational amplifier 703 and an offset correction circuit 704, where the offset correction circuit 704 has a capacitor 705 and switches 706 to 708. An input voltage Vin supplied from the outside is inputted to the non-inverting input terminal of the operational amplifier 703 via an input terminal 701 of the amplifier circuit. The output voltages Vout of the operational amplifier 703 are outputted to the outside via an output terminal 702 of the amplifier circuit.
The switches 706 and 707 are connected in series between the non-inverting input terminal of the operational amplifier 703 and the output terminal of the operational amplifier 703. The capacitor 705 is connected between a connection point of the switches 706 and 707 and the inverting input terminal of the operational amplifier 703. In addition, the switch 708 is connected between the inverting input terminal of the operational amplifier 703 and the output terminal of the operational amplifier 703.
Next, the operation of the amplifier circuit shown in FIG. 47 will be described by using the drawings. FIG. 48 is a timing chart showing the operation of the amplifier circuit shown in FIG. 47. As shown in FIGS. 47 and 48, first, only the switch 707 is in the on state and the other switches 706 and 708 are in the off state in a period T1 having a previous state. Thus, the output terminal and the inverting input terminal of the operational amplifier 703 are connected via the capacitor 705. In this state, the voltage level of the output voltage Vout is continued by a previous output voltage.
In a period T2, the switch 708 is on in addition to the switch 707. If the voltage level of the input voltage Vin changes, the output voltage Vout changes accordingly, and it becomes Vin+Voff including the offset voltage Voff. At this time, the capacitor 705 is short-circuited, and both ends of the capacitor 705 are at the same potential. In addition, the switches 707 and 708 are turned on so that both ends of the capacitor 705 are connected to the output terminal of the operational amplifier 703, and so the potentials of both ends of the capacitor 705 become Vout (=Vin+Voff) due to the output of the operational amplifier 703.
In a period T3, the switch 707 is turned off while keeping the switch 708 on, and thereafter, the switch 706 is turned on. Thus, one end of the capacitor 705 is connected to the input terminal, and the potential thereof changes from Vout to Vin. As the switch 708 is on, the potential of the other end of the capacitor 705 remains at the output voltage Vout. Therefore, the voltage applied to the capacitor 705 is Vout−Vin=Vin+Voff−Vin=Voff, and the capacitor 705 is charged by a charge equivalent to the offset voltage Voff.
In a period T4, the switches 706 and 708 are turned off, and thereafter, the switch 707 is turned on. As the switches 706 and 708 are turned off, the capacitor 705 is directly connected between the inverting input terminal and the output terminal of the operational amplifier 703 so that the offset voltage Voff is held by the capacitor 705. The switch 707 is turned on so that the offset voltage Voff is applied to the inverting input terminal of the operational amplifier 703 in reference to the potential of the output terminal. As a result of this, the output voltage Vout becomes Vout=Vin+Voff−Voff=Vin, and so the offset voltage is set off and the operational amplifier 703 can output a high-precision voltage.
However, as for the amplifier circuit shown in FIG. 46, it is necessary to constantly raise the potential of one end of the capacitor from a ground potential to the level of the input signal −IN. For that reason, there is a problem that it requires significant power consumption because it is accompanied by charge and discharge of the capacitor in an offset correcting operation.
On the other hand, as for the amplifier circuit shown in FIG. 47, a potential difference between both ends of the capacitor is only the amount of the offset voltage, so that the power consumption by charge and discharge of the capacitor can be lower than that of the amplifier circuit shown in FIG. 46.
However, the amount of the offset voltage generated to the operational amplifier is different according to the voltage level of the input signal. Moreover, fluctuation of the offset voltage due to the change in the voltage level of the input signal is the fluctuation in the units of mV. In the case where the amplifier circuit is used for a driving circuit for driving a liquid crystal display for instance, however, this fluctuation in the units of mV influences gray scale level display of the liquid crystal display. In particular, in the case where multiple gray scale level display and high-definition display are required by the liquid crystal display, it is essential to deal with the fluctuation of the offset voltage.
Therefore, in the case where the voltage level of the input signal supplied to the amplifier circuit shown in FIG. 47 changes in each output period, the amount of the offset voltage generated to the operational amplifier 703 changes in each output period, and so it is necessary to perform the offset correcting operation in each output period in order to realize high-precision output in the amplifier circuit shown in FIG. 47. If the offset correcting operation is performed in each output period, the capacitor for storing the offset voltage must be charged and discharged in each output period, and thus there is a problem that the power consumption on the offset correcting operation is significant even in the case of the amplifier circuit shown in FIG. 47.
In addition, if the offset correcting operation is performed by switch control, there is also a problem that output precision lowers due to influence of capacity coupling occurring on switching. This is because, as a parasitic capacity exists in an MOS transistor used for each switch, movement of a charge arises via the parasitic capacity on switching, and the charge equivalent to the offset voltage stored and held in the capacitor is influenced thereby. While it is possible to curb the lowering of the output precision occurring due to the influence of the capacity coupling on switching by increasing the capacity of the capacitor for storing the offset voltage, there is a problem that, if the capacity is increased, the power consumption increases due to the charge and discharge of the capacitor by the offset correcting operation performed in each output period.
While the problems of the amplifier circuits shown in FIGS. 46 and 47 were described above, the other amplifier circuits having offset correction means using the capacitor also have the same problems.
As the liquid crystal display has advantages of a low profile, light weight and low power, it is used for the display apparatuses of various types of equipment such as a note-sized personal computer. In particular, the liquid crystal display using an active matrix driving method is in increasing demand since it has advantages of allowing fast response, high-definition display and multiple gray scale level display.
A display portion of the liquid crystal display using an active matrix driving method generally has a semiconductor substrate on which transparent picture electrodes and thin-film transistors (TFT) are placed and an opposed substrate forming one transparent electrode on the entire surface, and is constituted by having these two substrates facing each other and inserting liquid crystal between them. And the TFT having a switching function is controlled to apply a predetermined voltage to each picture electrode, and transmittance of the liquid crystal is changed by the potential difference between each picture electrode and an opposed electrode provided on the opposed substrate so as to display an image. The semiconductor substrate has a data line for sending a plurality of level voltages (gray scale level voltages) to be applied to each picture electrode and a scanning line for sending a switching control signal of the TFT wired thereon, and application of gray scale level voltages to each picture electrode is performed via the data line. While various data line driving circuits have been used so far as a method of driving the data line, a representative example of the data line driving circuit thereof will be described below.
FIG. 49 is a diagram showing a configuration of a first data line driving circuit in the past. The driving circuit shown in FIG. 49 has a plurality of gray scale level voltages generated by a resistance string 421 impedance-converted by operational amplifiers 423-1 to 423-n (n is a positive integer) provided to the respective gray scale level voltages and has the voltages necessary for driving selected, of the impedance-converted gray scale level voltages, by selectors 422-1 to 422-m (m is a positive integer) and outputs them to a data line load so as to drive the data line. As this driving circuit has each of the plurality of gray scale level voltages generated by the resistance string 421 impedance-converted by the operational amplifiers 423-1 to 423-n, it has a high data line driving capability, and so it can increase a resistance value of the resistance string 421 for generating the gray scale level voltages and decrease the current running in the resistance string 421 so as to lower the power consumption of the driving circuit.
On the other hand, in the case of a large-sized liquid crystal display, it has a large number of data lines and the capacity of each data line is larger, so that a high driving capability is required of the data line driving circuit. As for the driving circuit in FIG. 49, there are the cases where a plurality of data lines are driven by one gray scale level voltage, and so it is short of the driving capability in case of being used for the large-sized liquid crystal display. Consequently, a second data line driving circuit in the past shown in FIG. 50 can be named as the data line driving circuit capable of obtaining sufficient driving capability even in case of being used for the large-sized liquid crystal display. The driving circuit in FIG. 50 has the gray scale level voltages necessary for driving selected, of the plurality of gray scale level voltages generated by a resistance string 421, by selectors 422-1 to 422-m, and has them impedance-converted by operational amplifiers 424-1 to 424-m provided to each data line as a data line output circuit, and outputs them to one data line load to apply a predetermined gray scale level voltage to each data line. As this driving circuit has the gray scale level voltages selected by the selectors impedance-converted by the operational amplifier provided to each data line, it has the sufficient driving capability even in case of being used for the large-sized liquid crystal display.
In recent years, portable apparatuses centering on a portable telephone and a personal digital assistant and so on are drastically becoming popular, and a mobile display is in highly increasing demand as the display apparatus for the portable apparatus. Although the capabilities required of the mobile display was centered on low power consumption in the past, the high-definition and multiple gray scale level display capabilities are also required in conjunction with diffusion of the portable apparatuses these days.
As for the liquid crystal display for performing the multiple gray scale level display, high output precision is required of the driving circuit because the potential difference between the adjacent gray scale level voltages is small. However, the driving circuit shown in FIG. 49 has a problem that, as each of the operational amplifiers 423-1 to 423-n has the offset voltage generated due to variations in characteristics of the transistors constituting the operational amplifiers, variations arise as to precision of the output voltage and display quality is lowered. The driving circuit shown in FIG. 50 also has the problem that, as each of the data line output circuits 424-1 to 424-m has the offset voltages generated as with the driving circuit in FIG. 49, the variations arise as to the precision of the output voltage and color shading occurs.
To solve this problem, there are the cases where each of the data line output circuits 424-1 to 424-m of the driving circuit shown in FIG. 50 uses the operational amplifier to which an offset correcting function is added. To be more specific, there are the cases where each of the data line output circuits 424-1 to 424-m of the driving circuit shown in FIG. 50 uses the amplifier circuit shown in FIG. 47.
In addition, the liquid crystal display for performing the high-definition display generally has the number of data lines which is larger than the number of gray scale levels, and so the driving circuit in FIG. 50 requires a large number of circuits since the data line output circuits 424-1 to 424-m are provided to m pieces of data lines. For that reason, there is a problem that the required area increases and the cost also increases.
In addition, also in case of using the amplifier circuit shown in FIG. 47 as each of the data line output circuits of the driving circuit shown in FIG. 50, it is necessary to provide the amplifier circuit shown in FIG. 47 to each of m pieces of data lines so that the required area increases and the cost also increases as to the liquid crystal display having a large number of data lines.
Furthermore, as for the driving circuit shown in FIG. 50, there are the cases where the voltage level of the input signal inputted to each data line output circuit is different in each output period. As mentioned above, if the voltage level of the input signal changes, the amount of the offset voltage generated to the operational amplifier also changes, so that this fluctuation influences the gray scale level display of the liquid crystal display. Therefore, in the case where each of the data line output circuits of the driving circuit shown in FIG. 50 uses the amplifier circuit shown in FIG. 47, the amount of the offset voltage generated to the operational amplifier 703 in each output period changes as the voltage level of the input signal to each amplifier circuit changes in each output period, and so it is necessary for each amplifier circuit to perform the offset correcting operation in each output period in order to realize the high-precision output in each amplifier circuit and thereby realize the high-precision display and multiple gray scale level display in the liquid crystal display. However, there is a problem that, if the offset correcting operation is performed in each output period, the capacitor for storing the offset voltage must be charged and discharged in each output period and thus the power consumption increase.
In addition, the offset correcting operation is performed by switch control, and so there are the cases, as mentioned above, where the output precision of each amplifier circuit lowers due to the influence of capacity coupling occurring on switching. If the capacity of the capacitor is increased to curb the lowering of the output precision, there is a problem that, the power consumption increases due to the charge and discharge of the capacitor by the offset correcting operation performed in each output period.
Moreover, Japanese Patent Laid-Open No. 2001-100704 describes a technology of, by providing a plurality of resistances for adjustment to a resistance dividing circuit for dividing the voltage of liquid crystal driving power, reducing the offset voltage of each amplifier according to the size of the resistance to enhance the output precision. However, there are variations in the resistances themselves in the first place, and so the offset voltage of each amplifier cannot be sufficiently reduced according to the size of the resistance even if so attempted, so that sufficient output precision cannot be obtained.